Electromagnetic energy may be employed to facilitate examination of the composition of an unknown gas via photochemistry applications such as soft ionization and photo-fragmentation. The vacuum ultraviolet (VUV) region of the electromagnetic spectrum is particularly useful in these applications because the energies of VUV photons (generally 6-124 eV) correspond to electronic excitation and ionization energies of most chemical species. Vacuum ultraviolet (VUV) light is generally defined as light having wavelengths in the range of 10-200 nanometers.
Most existing systems involve generating VUV light remotely from the area to be exposed, for example using a resonance lamp, frequency-multiplied laser, or synchrotron, and attempting to deliver this light to the area of interest, typically by passing the VUV light through a window. However, window materials and refractive optics in this wavelength range are scarce or non-existent, so it is often impractical to direct or concentrate VUV light. The windows that are employed typically absorb a large fraction of light in this wavelength spectrum, and reflective optics can become contaminated in a less-than perfectly clean environment. In addition, lasers and synchrotrons can be prohibitively expensive and can require large amounts of power and space.
So-called “windowless” photoionization devices (“ionization devices”) allow a greater portion of the light spectrum to be incident on a sample. However, in known windowless ionization devices, positive ions of the plasma (“plasma ions”) and electrons of the plasma (“plasma electrons”) can travel through the aperture through which the light of the plasma is desirably transmitted. The presence of the plasma ions in the ionization region can result in interfering peaks with analyte ions of the sample, and ultimately reduce the reliability of the detection of analyte ions of interest. Plasma electrons and ions can undesirably give rise to hard ionization of the analyte ions of the sample in an uncontrolled manner, either through electron impact ionization or ion-molecule charge transfer reactions.
What is needed, therefore, are better systems and methods of generating VUV light and delivering the VUV light to an area of interest.